This invention relates in general to water treatment systems and, more specifically, to a method and apparatus for removing volatile organic contaminants from water.
Treatment systems using a variety of technologies have been developed over the years to remove a variety of contaminants from water supplies, both to treat drinking water prior to use or sewage prior to reintroduction into the environment. A number of different techniques, usually involving flotation, are used to remove solid particulate material. Other methods have been developed to remove naturally occurring gaseous contaminants such as sulfur-containing gases from water. Today, many wells and other water sources have become contaminated with volatile organic compounds, such as trichloroethylene (TCE), tetrachloroethylene, petroleum hydrocarbons, and benzene, or mixtures of such compounds.
The traditional method for removing gaseous contaminants has been the packed tower air stripping method. A vertical tower, usually on the order of 20 to 30 feet high, is constructed to treat the water at atmospheric pressure. Contaminated water is pumped to the top of the tower and is allowed to cascade down over packing or plates within the tower. Air is forced upwardly through the tower by a fan to volatilize any volatile organic contaminants. The contaminant-free water is collected at the bottom of the tower and is pumped into the distribution system. The air exiting the top tower may be vented to the atmosphere or collected, dehumidified and purified (usually with activated charcoal filters) before release to the atmosphere.
While generally effective, these towers have a number of disadvantages. The towers are expensive to construct, operate and maintain. Operation is expensive because great quantities of air must be forced up against the falling water to achieve the required level of treatment and the energy needed to pump the water to the top of the tower is lost with the cascading and further energy is expended in pumping the decontaminated water back to the distribution system pressure. The high capacity fans, pumps and motors tend to require high levels of maintenance. Also, the tall towers have an adverse visual impact on the surrounding area, which may be particularly important in residential or commercial areas.
A number of other water treatment systems have been developed using air jets or bubbles to help remove contaminants, both solid and gaseous, from water. Some are flotation systems in which are bubbles are introduced into contaminated water in a manner permitting the bubbles to attach to fine solid particles and float to the top, where the accumulated foam-like layer can be skimmed off. Typical of these is the process described by Treyssac in U.S. Pat. No. 4,146,472. There, two separate flows of liquid are separated by a fine mesh grid, with particulate contaminated water above the grid and a higher pressure air saturated layer of water below the grid. Small bubbles pass upwardly through the grid and capture fine particles as they move to the surface for skimming. While this arrangement may effectively remove particulates, there is nothing to suggest removal of dissolved volatile organic contaminants. Further, this arrangement is complex, requires precise sizing and pressure relationships to prevent intermixing of flows and turbulence and is energy intensive in requiring large volumes of high pressure air-saturated water.
Various filter arrangements have been developed for removing inorganic gases, such as sulfur-containing gases, and particulates from well water or the like. Typical of these is the filter described by Kemper in U.S. Pat. No. 4,094,789. There, high pressure air and water jets are positioned to impinge on a splash plate, causing immediate and violent co-mingling of the air and water, causing at least some of the sulfur gases to mix with the air and be carried away therewith. The water then passes through a granular filter for particulate removal. This is very low volume, high energy requirement system, requiring low volume, high velocity jets of water and air.
Increasingly, water supplies, particularly wells, have been becoming contaminated with a variety of volatile organic compounds which are by-products of high technology industries. While these compounds are often present in only very small proportions, because of the high toxic or carcinogenic characteristics of some of them removal from drinking water supplies is essential.
Prior water treatment methods are often ineffective in removing volatile organic contaminates from water. Prior methods also tend to be complex, costly and either very low in capacity or require very large plants for effective utilization. Thus, there is a continuing need for an improved method and apparatus for removing volatile organic contaminants from water which can be efficiently sized for both large and small volume operations, is cost and energy efficiently and is environmentally non-intrusive.